This invention relates to the field of high-speed laser illuminated, shutterless photography wherein precise time positioning of laser light pulses with respect to the photographic film motion is important.
In the study of burn characteristics of solid state rocket grain or propellant, interesting conflicts occur and tradeoff compromises are required between the burning characteristics of the fuel grain and the properties of the apparatus employed to study the grain burn phenomenon. Generally one of these conflicts pits the limited depth of field of a photographic optics system against the rapid and irregular time progression of the grain burn surface. Another of these conflicts involves possible similarity between the color spectrum of the photographic illumination and the incandescent gases of the propellant sample combustion. An enabling improvement with respect to this second conflict, however, is found to reside in the use of a monochromatic laser light source and complementing filters during a photographic event--in order that the laser spectral energy be narrowly confined and usably segregated from the spectral bands present during combustion of the fuel grain. The ability to achieve reliable and accurate synchronization between the short duration laser illuminating pulses and the moving frames of the photographic film exposed with such lasers, is however, also a conflict area requiring some consideration in the achievement of quality burn face photography.
The metal vapor lasers which are preferred for generating monochromatic illumination of a fuel grain burn face -- such lasers as the copper-vapor laser, inherently add additional complexity to the execution of high-speed motion picture studies. In particular, the necessity of maintaining such lasers in continuous operation in order to achieve stable output energy levels and spectral content, together with the susceptibility of such lasers to double triggering short cycle operation requires detailed consideration in the assembly of a burn face photography system. Furthermore, in the arrangement of such photographic systems it is usually desirable to synchronize the light-supplying laser with the film motion in a master-slave relationship. This relationship is especially desirable where the slower, more inertia-prone photographic camera can be the master apparatus and the more flexible electronic circuit incorporating laser is arranged as the slave apparatus.
As evidenced by the following identifying patents, a considerable degree of inventive activity has been devoted to the satisfactory operation of high-speed photographic systems including such systems operating with the use of a pulsed laser illumination source.
Synchronization of a light source with the operation of a photographic camera system is found in the patent of A. E. Lennert et al, U.S. Pat. No. 3,671,112, which concerns an arrangement for applying a time reference scale to high-speed film in a high-pulse repetition rate environment. The Lennert et al patent is especially concerned with overcoming limitations encountered in using a neon lamp to expose timing reference marks on high-speed motion picture film. To this end, the patent addresses the problems of delay line capacitance, neon gas ionization times, and the transmission of signals through unterminated transmission line segments. In resolving the noted difficulties with neon lamp exposure of film, the Lennert et al apparatus uses a small neon keep-alive current and employs characteristic impedance terminated transmission lines and drivers. The Lennert et al patent is, however, not concerned with synchronized operation of a laser device nor with the constant temperature self-heat requirements of laser equipment.
Another prior patent concerned with illumination and photography is in the name of Hubert Guillet et al, U.S. Pat. No. 3,643,568. The Guillet et al apparatus accomplishes synchronization of a photographic light-supplying laser with the film transport mechanism of a photographic camera by way of slaving the light-supplying laser and the camera both to a second low-powered laser. The high-powered light-supplying laser in the Guillet et al apparatus is triggered by light received from, for example, a small helium-neon laser, while movement of the camera is initiated in response to an electrical signal generated from light supplied by the same triggering helium-neon laser. The Guillet et al apparatus employs a plurality of mirrors, one rotating in nature, for communicating light signals to a photomultiplier tube and to the input surface of the high-powered laser. The Guillet et al apparatus also employs a micrometric screw apparatus for adjusting the time position of the laser generated energy bursts.
A film speed control apparatus for use in a high-speed motion picture camera is also shown in the patent of Gordon W. Brant, U.S. Pat. No. 4,405,217, which concerns a driving circuit and speed control arrangement for a prism type high-speed motion picture camera--a Hycam.RTM. camera of the type shown in U.S. Pat. Nos. 3,523,722 and 3,259,448, in particular. The Brant apparatus concerns a silicon-controlled rectifier circuit capable of providing camera motor protection, smooth film starts, film damage protection, and other drive characteristic features. The Brant apparatus is however concerned with the synchronizing of laser pulses with film movement in the camera.
Another patent, U.S. Pat. No. 3,987,468 issued to Soichiro Matsuzaki et al, concerns flash synchronizing and flash enabling and disabling circuitry for use in a stationary camera of the 35 millimeter type. The Matsuzaki et al patent is also not concerned with laser illumination or high-speed motion picture photography.
A circuit suitable for use in exciting a Kerr cell employed as the shutter of a high-speed camera is disclosed in the patent of Wilbur A. Hane, U.S. Pat. No. 2,783,376. The Hane invention concerns a vacuum tube electronic circuit usable for providing the signals needed in switching a Kerr cell optical element between energy transmitting and blocking optical states.
The patent art also includes the patent of W. E. Buck et al, U.S. Pat. No. 3,366,439, which concerns a laser illumination and shuttering arrangement for high-speed photography. In the Buck et al patent, an argon gas pulsed laser is used in conjunction with a rotating mirror and a slitted stop plate for illuminating and photographing workpiece objects. The slitted stop plate in the Buck et al invention provides improved rise and fall times for the workpiece illumination. The Buck et al apparatus also contemplates photographic recording of a self-luminous event and employs monochromatic laser illumination with filters for suppressing the non-laser illumination.
These patent references indicate the combination of electronic circuitry, high-speed motion picture cameras, and laser illumination sources to be desirable. Nevertheless, the examples provided by the above patents have been unable to achieve an optimum laser-to-camera synchronization system which also accommodates certain laser operating peculiarities.